Plume/Crest Colors?

[Robert Vermaat]

what do u think ?

Well the difference between infantry and cavalry are clear: they who sit on a horse are cavalry.

Besides that, differences do not matter. 
You are thinking about a modern army with uniforms centuries before anything like that existed.
The Roman army was something the world had never seen before (well, in Europe) but it was still an Iron Age society and modern ideas would have been very alien to them. 

So, unlike the modern US Army where cavalry and infantry are more like army classes which can both have (for example) armoured vehicles, in the Roman army cavalry and infantry were troop types. No need to distinguish between them, everyone could see the difference. 

Marine troops (naval infantry is different from modern Marines, again ancient types and modern classes) are in one source described to have been wearing 'sea-blue' tunics, but it seems that this 'blue uniform' may be a bit overdone these days by reenactors. 

Generals could have plumes in whatever colors they wanted - it was more a question of bling-bling than (again) a uniform.